1. Field of the Invention
The present invention relates to a cathode-ray tube for color display used in a color display device which is a terminal unit of a computer, said cathode-ray tube being capable of applying an input to the computer by the operation of a light pen.
2. Description of the Prior Art
A display device for entering information by using a light pen from a cathode-ray tube to a display control device including a computer is known in the art. The cathode-ray tube in such a conventional display device is used for monochromatic display. It comprises a luminescent screen coated with a mixture of a fluorescent material having a predetermined luminescent color and a short afterglow fluorescent material having a ten percent afterglow time less than 50 microseconds, so that communication with the display device can be established by the operation of a light pen. Recently, color display devices have been often used, and there has been a demand that color display devices have also such a communication function with a light pen as in monochromatic display devices.
A luminescent screen of a cathode-ray tube for color display used in a color display device includes generally three picture elements, which differs from the case of a cathode-ray tube for monochromatic display. A picture element layer comprising these three picture elements is formed by a photographic printing method or the like and accordingly a far greater amount of fluorescent materials is required as compared with the case of a cathode-ray tube for monochromatic display in order that a light pen can be used. In addition, aside from the amount of fluorescent materials, it is not easy at the time of manufacturing cathode-ray tubes for color display to apply a coating discriminately for those not requiring light pen operation or those requiring such. More specifically, if a fluorescent material for each picture element is mixed with a fluorescent material having a ten percent afterglow time less than 50 microseconds to make a photosensitive slurry for coating by a photographic printing method for the purpose of improving an operational function of a light pen, such a slurry cannot be used for forming a luminescent screen which does not require light pen operation. The reason is that even if the same fluorescent material is used, the color of an ultrashort fluorscent material, that is, a fluorescent material having a 10 percent afterglow time less than 50 microseconds differs from the original color and its brilliance is decreased and as a result the luminescent color and brightness are deteriorated. Specifically speaking, as an ultrashort afterglow fluorescent material, P16, P31, P24, P36, P38, P48, P46, P47 or the like are conventionally used. For example, if P46 (Y.sub.3 Al.sub.5 O.sub.12 :Ce) is used to form a luminescent screen by mixing each luminescent picture element of the three primary colors red, blue and green with five percent weight of P46 for coating, the luminescent brightness of each luminescent picture element is decreased by approximately 10% and the luminescent color of each picture element is perceptibly changed with a bluish color caused by the P46. Accordingly, in order to manufacture a luminescent screen of an unchanged color which does not require light pen operation, it is necessary to prepare a photosensitive slurry not including P46 for each color, which causes a complication of a manufacturing process and decreases considerably the manufacturing efficiency.